This invention relates to a differential amplifier and, more particularly, to a complementary differential amplifier circuit with source-follower driving circuits.
In order to increase an input impedance of a differential amplifier comprised of bipolar transistors, source-follower driving circuits may be connected to the bases of differential bipolar transistors. With such a differential amplifier, if the drain-to-source voltage V.sub.DS of field effect transistors used in the source-follower driving circuits is fixed substantially constant irrespective of variations in supply voltage, then it will be possible to prevent distortion of a gate input signal which is caused by the variation in the input impedance resulting from the variation in gate-to-drain capacitance C.sub.rss which is caused by the variation in the drain-to-source voltage V.sub.DS and improve the stability of the differential amplifier against the variation in supply voltage, that is, the supply voltage rejection ratio (S.V.R.R.). A prior art differential amplifier, as shown in FIG. 1, arranged to hold the drain-to-source voltage V.sub.DS of source-follower field effect transistors substantially constant is known.
In the known differential amplifier of FIG. 1, to the bases of bipolar differential transistors 1 and 2 having their emitters coupled together are connected source-follower field effect transistors 3 and 4, respectively. The drain-to-source voltage of each of the field effect transistors 3 and 4 can be kept constant because of a constant voltage drop across a resistor 5 connected to the emitters of differential transistors 1 and 2, and a negligible variation of the base-to-emitter voltages of the transistors 1 and 2 which are caused by amplification of a signal, even if a supply voltage (between +V and -V) fluctuates. The variations in the gate-to-drain voltage of each of the field effect transistors 3 and 4 are also negligible because the variations of the gate-to-source voltage of each of the field effect transistors 3 and 4 are negligible. Therefore, the circuit of FIG. 1 can improve the supply voltage rejection ratio and eliminate or reduce the distortion which is caused by variations in the drain-to-gate capacitance of the field effect transistors 3 and 4. Since such distortion as described above cannot be reduced by negative feedback in the differential amplifier used as a first-stage amplifier adapted for signal amplification, the reduction of distortion is very advantageous.
The prior art circuit of FIG. 1 has, however, a circuit configuration which is asymmetrical with respect to positive and negative supply voltages (+V, -V). Therefore, it is very difficult to derive output signals which are symmetrically biased to positive and negative DC voltages of a certain magnitude, restricting the flexibility of design for a subsequent amplifier stage.